This invention relates generally to turbine blades and, more particularly, to blade structures with improved cooling.
Turbine blades employed in gas turbines include a leading edge and a trailing edge. The leading edge is the blade surface that is first contacted by the working medium gases in the turbo-machine. The trailing edge is the blade surface that is last contacted by the working medium gases as the gases pass by the blade.
The temperatures within gas turbines may exceed 2500 degrees Fahrenheit, and cooling of turbine blades is very important in terms of blade longevity. Without cooling, turbine blades would rapidly deteriorate. Improved cooling for turbine blades is very desirable, and much effort has been devoted by those skilled in the blade cooling arts to devise improved geometries for the internal cavities within turbine blades in order to enhance cooling.
With respect to blade cooling, some known turbine blades have internal cavities forming a serpentine cooling circuit. Turbulators, sometimes referred to as ribs, are cast on the suction and pressure side passage walls of the serpentine cooling circuit. The ribs are usually cast with a rib height to passage hydraulic diameter ratio of 0.1 to 0.2 and are positioned perpendicular, or at an angle, to the cooling medium flow. The cooling media usually is fed at the root section of the blade and exits either through blade tip and trailing edge openings, or through film holes located between two adjacent turbulators. In a closed circuit cooling system, the cooling media is returned to the root exit region of the blade.
The walls defining the serpentine cooling circuit are smooth and do not provide high cooling rates, i.e., high heat transfer coefficients. The ribs, however, enhance the coolant side heat transfer. Particularly, the ribs break the hot thermal boundary layers near the side walls, thus causing separation and reattachment regions which cause strong mixing between the hot coolant near the walls and the relatively lower temperature coolant flowing in the center portion of the passage.
Although the above described ribs provide improved cooling, it would be desirable to provide even better blade cooling. In providing even better blade cooling, it also would be desirable to avoid significantly increasing blade fabrication costs.